feat: the apps' icons as one family, drawn by one tool (ADR 0031)

tool/app_icons.py at the root replaces Learn's and Todo's scripts: each
app's Lucide glyph (lucide-static 1.50.0) drawn by librsvg, fading from
cobalt through the colours ADR 0031 names, then every platform's
launcher icon through flutter_launcher_icons, Windows' .ico with every
size, and Learn's snap icon.

Learn's icon becomes the graduation cap, its window's on Linux too, with
Lucide's notice in its licence list; the kingfisher stays the emblem on
the rail and the launch screen. Todo's becomes list-todo, Train's bike.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/lib/src/app/licences.dart

@@ -2,11 +2,11 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
 /// Where Lucide's notice is: its licence, verbatim from the release the app's
-/// icon was drawn from (`../tool/app_icons.py`).
+/// icon was drawn from (ADR 0031).
 const lucideLicenseAsset = 'assets/icon/LICENSE-lucide.txt';
 
 /// Adds Lucide's notice (ISC) to the app's licence list: the app's icon is
-/// Lucide's `list-checks`, and its licence asks that the notice go with it.
+/// Lucide's (ADR 0031), and its licence asks that the notice go with it.
 void registerLucideLicense() => LicenseRegistry.addLicense(() async* {
   yield LicenseEntryWithLineBreaks(const [
     'Lucide (the app icon)',